If you collaborate with people who use Excel, it is better for you to use Excel. Importing into Numbers changes the formatting of the document and may remove formulas that don't import (replacing the formula with the last calculated value). Exporting back to Excel changes it again. What you send back to them will likely not look like what they sent you.
To answer your question, in Numbers use the menu item Numbers/Export to. Choose Excel. Then you have to choose whether you want each table to become an Excel worksheet or you want each sheet (which might have multiple tables) to become an Excel worksheet.